1. Field of the Invention
The present invention relates to a system for measuring the residual stress of optical fibers, and more particularly to a system for measuring the residual stress remained in an optical fiber and has an asymmetrical circular stress distribution.
2. Description of the Related Art
In general, a residual stress created during the manufacturing stage of drawing an optical fiber remains within the optical fiber. This residual stress causes undesirable changes in the refractive index of the fiber and, as a result, increases the light loss in the optical fiber caused by a photoelastic effect. As such, the residual stress and its distribution in the optical fiber must be measured accurately and then adjusted accordingly.
The residual stress in the optical fiber is typically measured using a photoelastic effect, which is a phenomenon in which a refractive index of the fiber is changed along a direction of the stress remaining in a transparent solid medium, such as an optical fiber. Thus, the refractive index of an optical fiber or an optical fiber pre-form changes along the polarized direction, and this change can be observed to measure the stress remained in the fiber.
FIG. 1 is a schematic view of a conventional system used for measuring the residual stress of optical fibers, and FIG. 2 is a diagram showing a phase shift along a light path.
As shown in FIG. 1, the conventional residual stress measuring system includes a light source 10 such as a laser, a lens system 30 for aligning the generated light, a polarimeter 50 for polarizing the aligned light, a residual stress measuring section 60, and a light detector 70 for measuring a phase shift of the light generated from a photoelastic effect, which is caused by the residual stress remained in the fiber.
In operation, light cast from the light source 10, such as a Helium-Neon laser, is reflected by a mirror 20 and passes through the lens system 30, which includes a beam diffuser 31, a lens 32, and a iris 33, and then is converted into a plane wave. The plane wave is reflected again by another mirror 40 and enters into the polarimeter 50, which includes a polarizer 51, a wavelength plate 52, and a condenser 53. The functions of the mirrors 20 and 40 are to provide a compact design by changing the light path within a given space. The polarized light is incident into an optical fiber 62 positioned in the residual stress measuring section 60, which includes a slide glass 61 and a cover glass 63. The light passing through the optical fiber has a phase difference resulting from a stress distribution of interior of the optical fiber, and further passes through an objective lens 71 and a polarization analyzer 72. A sensor 73 transforms the intensity of light passing through the polarization analyzer 72 into an electrical signal, thereby measuring the residual stress of the optical fiber.
The residual stress of the optical fiber can be represented by a formula below.I(y)=Io sin 2(Φ(y)/2),wherein Io represents the intensity of a background, and Φ(y) represents the phase shift induced by the residual stress.
As shown in FIG. 2, it will be seen that light incident on the fiber and in parallel with an x-axis may be divided into two components. A phase shift of the incident light can be represented as follows:       ⅆ          Φ      ⁡              (        y        )              =                    2        ⁢                                   ⁢        π            λ        ⁢          ⅆ              x        ⁡                  (                                    n              y                        -                          n              z                                )                    
Therefore, the phase shift generated by the residual stress can be represented by a photoelastic effect, which is illustrated mathematically as follows:ny−nz=C(σy−σz)and       ⅆ          Φ      ⁡              (        y        )              =                    2        ⁢                                   ⁢        π        ⁢                                   ⁢        C            λ        ⁢                  ⅆ                  x          ⁡                      (                                          σ                y                            -                              σ                z                                      )                              .      
Further, total residual stress and the photoelastic effect have a profile that can be represented as follows:                                           σ            z                    ⁡                      (            r            )                          =                                            -              λ                                      2              ⁢                                                           ⁢                              π                2                            ⁢                              C                o                                              ⁢                                    ∫              r              b                        ⁢                                                                                ⅆ                    Φ                                    /                                      ⅆ                    y                                                                                                              y                      2                                        -                                          r                      2                                                                                  ⁢                              ⅆ                y                                                                    and                                                              C              ⁡                              (                r                )                                      ⁢                          E              ⁡                              (                r                )                                              =                                                    -                λ                                            2                ⁢                                                                   ⁢                                  π                  2                                                      ⁢                                          ∫                r                b                            ⁢                                                                                          ⅆ                      W                                        /                                          ⅆ                      y                                                                                                                          y                        2                                            -                                              r                        2                                                                                            ⁢                                  ⅆ                  y                                                                    ,                            wherein Co represents the photoelastic coefficient of the fused silica. Note that these formula are applied under conditions as follows:        
1. An incident light must pass through the interior of the optical fiber on a straight direction without changing its path.
2. A Brewster coefficient must be constant in an incident direction, i.e., in a diametrical direction (a double refraction does not occur due to a constant composition characteristics of the materials, which allows a phase shift to be generated by the residual stress).
3. Residual stress induced by a radius or an angle may not be ignored over the axial stress (σz>>σθ,σp).
As illustrated above, it is possible to measure the residual stress of an optical fiber only in a single direction according to teachings in the art. Therefore, it is difficult to precisely measure the residual stress and its distribution in an optical fiber and, in particular in an optical fiber having an asymmetrical circular stress distribution or having regularly spaced air holes along the fiber cladding (known as a photonic crystal fiber).